Patent GB-1432609-A (GB)
Title: Interferometer spectrometer for discrete frequency analysis of  emission or absorption spectra and method
IPC: G01J1/42
Filed: 1973-06-13
Abstract: 1432609 Interference spectrometers; gas analyzers EOCOM CORP 13 June 1973 [23 June 1972] 28071/73 Heading G1A A spectrometer includes an interferometer 60, Fig. 1, having a moving mirror; light to be investigated is passed to the interferometer where an interferogram is produced and an electric signal is formed by focusing the light on transducer 106. This signal is an inverse Fourier transform of the transducer output spectrum for finite mirror movement, and is sampled and held at 126 at regular increments of mirror movement; the sampled signal is converted to a digital signal at 128 and then fed to one or more digital Fourier series frequency component synthesizers 130 which each give an output representing the amplitude of the input light at a respective predetermined frequency. Because the sampling is effected at regular increments of the mirror movement it is necessary to accurately know the mirror position. To monitor the mirror, coherent light from laser 70 is also passed into the interferometer, converted to an electric signal at 116 whose zero crossings occur at-equal increments of mirror movement, Fig. 10 (not shown), and are used to trigger the sampling via pulse generator 124. The spectrometer may be used to measure the amount of specified pollutants in exhaust gases. Each pollutant has a frequency at which it characteristically absorbs light, so that if broad band light is passed through the gas under test and into the spectrometer the Fourier synthesizers will show how much light is present at each characteristic frequency. By comparing this with the synthesizer outputs when the gas is not present the amount of light absorbed is found, and hence the concentration of the pollutants. The spectrometer may also be used to analyze light, e.g. from a star. Source: the broad band source is shown in Fig. 2, and comprises a black body radiator 43, e.g. tungsten, heated by current supplied on leads 46. Radiation emerges through pinhole 45 and a window 42 of the housing 41. The housing is preferably evacuated and may be internally reflective. The window is preferably of sapphire since, for exhaust gas analysis, this allows the passage of infrared. Absorption cell: the cell is shown in Fig. 3, and comprises a casing 66 with a gas inlet 52 and outlet 78. The inlet opens into a manifold 54, and glass wool 56, 76 with honeycombs 60, 74 ensure laminar flow through the test region 62. Curved mirrors 64, 68, 70 provide a folded optical path through the cell between two windows (72), (73) in the side wall of the cell as in Fig. 4 (not shown). To prevent contamination of the mirrors clean air supply pipes 69 form a curtain of air at the cell edges. Interferometer: the interferometer, Fig. 5 (not shown), comprises a Michelson arrangement having a beam-splitter at 45 degrees to the incoming light. The light is directed from this to a fixed mirror, and also passes through the beam-splitter to the moving mirror, the reflections from the two mirrors interfering to form an optical output signal with various frequency components. A filter, e.g. of germanium, is provided in the path of the optical output to limit the highest frequency thereof. This is done because for proper sampling the highest optical frequency must be less than the laser frequency. Particle separation: for exhaust gas analysis it is necessary to remove all particles having a size greater than one tenth of the light wavelength.  This is done by passing the gas through a filter which comprises a centrifuge. Two centrifuges are described, Figs. 14-17 (not shown), in both of which the gas to be filtered is passed along a hollow shaft to which vanes are attached, shaft rotation causing the particles to move to the outside circumference of the device. The particles may be collected for analysis.